A quantum well designed to detect infrared (IR) light is called a quantum well infrared photodetector (QWIP). QWIPs operate by photo-excitation of electrons between a ground state and an excited state of its quantum wells. In more detail, a quantum well absorbs IR photons. This absorption of IR photons photo-excite electrons from the ground state to the excited state of each quantum well. The excited states of the quantum wells making up a QWIP lie close to or within an energy transport band (sometimes referred to as the continuum or a miniband). A voltage externally applied to the QWIP operates to sweep out the photo-excited electrons, thereby producing a photocurrent in the continuum or miniband.
Quantum wells are grown in a crystal structure. In general, layers of two different, high-bandgap semiconductor materials are alternately grown. The bandgap discontinuity of the two semiconducting materials creates quantized sub-bands in the wells associated with conduction bands. Only photons having energies corresponding to the energy separation between the ground and excited states are absorbed. This is why a QWIP has such a sharply defined absorption spectrum. Note that each well can be shaped to detect a particular wavelength, and so that it holds the ground state near the well bottom, and the excited state near the well top.
A group of QWIPs can be used to form a focal plane array (FPA) in detection applications, where each QWIP effectively acts as a pixel of the array. Each QWIP structure is designed to produce a signal that is transmitted to a read out circuit. The group of the signals from all the pixels of the FPA can be used to produce an image corresponding to the received infrared radiation. In a multicolor application, the QWIP structures must be configured so that each desired color can be detected. Signals produced by the QWIPs must then be captured and processed so that images can be formed and displayed. Such functionality is not trivial, and is associated with a number of difficult implementation problems.
In addition, although various tunable absorption schemes have been developed, where the wavelength of absorption is tunable (e.g., such as those using asymmetric quantum wells), there appears to be no device that is capable of tunability at the FPA level for performing the likes of target imaging. In particular, it would be highly desirable to have a multicolor, multifocal plane optical detector device that has a spectral response tunable at both a coarse tuning level (e.g., switch between two spectral bands, such as from mid-wavelength to long-wavelength) and a fine tuning level (e.g., within one spectral band, such as from mid-wavelength to mid-wavelength).
The tunable detector would ideally be a monolithic or unitary device that can be fabricated by conventional epitaxial growth techniques on a single substrate with a sufficient number of contacts suitable for direct, discrete, pixel to readout integrated circuit (ROIC) connection and direct current readout for each detected wavelength, and that can be easily scaled up to large array configurations suitable for the many applications for which such a device would be attractive.
What is needed, therefore, is a QWIP FPA design that is configured for spectral tunability for performing the likes of imaging and spectroscopy.